Evoked Stapedius Reflex Threshold (ESRT) Tile Electrode

ABSTRACT

An electrode arrangement is described for sensing electrical activity in target tissue. An inner electrode has an elongate electrode body formed as a cylindrical section with an inner penetrating end for insertion into the stapedius muscle target tissue. An outer electrode fits over the inner electrode and an outer penetrating end for insertion into the target tissue. The two electrodes are joined together with their electrode bodies in parallel so that the penetrating ends of the electrodes penetrate in the same direction into the target tissue to sense electrical activity in the target tissue.

This application claims priority from U.S. Provisional PatentApplication 61/423,717, filed Dec. 16, 2010; which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to an electrode configuration formeasuring the action current and/or the action potential of electricallyactive tissue, specifically a bipolar stapedius muscle electrodeconfiguration for measuring the action potential generated upon acontraction of the stapedius muscle.

BACKGROUND ART

The human ear may be divided into the outer ear, middle ear, and innerear. The middle ear includes the eardrum and the auditoryossicles—hammer, anvil, and stirrup. Sound waves entering the outer earcause the eardrum to oscillate. A mechanical impedance conversion occursin the middle ear, which allows an optimum transmission of the soundsignal from the outer ear to the inner ear. Thus, the ear drumoscillations are transmitted by the ossicles to the oval window of theinner ear which vibrates the fluid within the cochlea. Hair cellsprojecting into the cochlea are bent by the vibration of the cochlearfluid, thereby triggering nerve pulses.

The middle ear also contains the tympanic muscle and the stapediusmuscle. The tympanic muscle is linked to the hammer, the stapediusmuscle being connected via a tendon to the stirrup. In case of anexcessively high sound pressure which could damage the inner ear, bothmuscles contract reflexively to decrease the mechanical coupling of theeardrum to the inner ear (and thereby also the force transmission). Thisprotects the inner ear from excessively high sound pressures. Thistensing of the stapedius muscle when triggered by high sound pressuresis also referred to as the stapedius reflex. Medically relevantinformation about the functional capability of the ear may be obtainedfrom the diagnosis of the stapedius reflex. The measurement of thestapedius reflex also is useful for setting and/or calibrating cochlearimplants, because the sound energy perceived by a patient may be deducedfrom the measured stapedius reflex.

The stapedius reflex can be determined in a post-operative clinicalsetting using an acoustic tympanometer which also requires anotheradditional device to take and use the electrical measurements. Tomeasure the stapedius reflex, it is known to intra-operatively useelectrodes that are brought into contact with the stapedius muscle torelay to a measuring device the action current and/or action potentialsgenerated upon a contraction of the stapedius muscle. A reliableminimally-invasive contact of the stapedius muscle is difficult becausethe stapedius muscle is situated inside a trough present in a bone andonly the tendon of the stapedius muscle connected to the stirrup and itsupper part are accessible from the interior of the middle ear.

Various intraoperative stapedius muscle electrodes are known from U.S.Pat. No. 6,208,882, however, these only achieve inadequate contact ofthe stapedius muscle tissue (in particular upon muscle contraction) andare also very traumatizing. In order to make ESRT measurements simplerand quicker, first non-commercial intraoperative experiments and studieshave been conducted with monopolar (Almqvist et al. 2000) or bipolarhook electrodes (Pau et al. 2008), respectively, which have beenattached at the stapedius tendon or muscle to measure the muscleactivity in the case of a reflex. The measurements were successful, butthe electrode design was only suitable for intra-operative tests. TheAlmqvist hook electrode does not allow a quick and easy placement at thestapedius tendon and muscle—the electrode has to be hand held duringintra-operative measurements. the Pau bipolar hook electrode does notensure that that both electrodes are inserted into the stapedius muscledue to the small dimensions of the muscle and the flexibility of theelectrode tips. One weakness of these electrodes is that they do notqualify for chronic implantation.

DE 10 2007 026 645 A1 discloses a two-part bipolar electrodeconfiguration where a first electrode is pushed onto the tendon of thestapedius muscle or onto the stapedius muscle itself, and a secondelectrode is pierced through the first electrode into the stapediusmuscle. One disadvantage of the described solution is its rathercomplicated handling in the very limited space of a surgical operationarea, especially manipulation of the fixation electrode. In addition,the piercing depth of the second electrode is not controlled so thattrauma can also occur with this approach.

U.S. patent application Ser. No. 12/763,374, filed Apr. 20, 2010(incorporated herein by reference) describes an electrode arrangement100 as shown in FIG. 1 for sensing electrical activity in target tissue.A support electrode 101 has an elongate electrode body with a base end102 and a penetrating end 103 for insertion into the target tissue. Afixation electrode 104 has an elongate electrode body with a base end105 and a penetrating end 106 at an angle to the electrode body. Thepenetrating electrode 104 passes perpendicularly through an electrodeopening 107 in the support electrode 101 so that the penetrating ends103 and 106 of the electrodes penetrate into the target tissue so thatat least one of the electrodes senses electrical activity in the targettissue.

It would be advantageous to have a simple cost effective electrode formeasuring action currents and/or action potentials in electricallyactive tissues (such as the stapedius muscle tissue), which enablessecure but reversible fixing of the electrode in the target tissue, butwhich traumatizes the tissue as little as possible.

SUMMARY

Embodiments of the present invention are directed to an electricallyelicited stapedius reflex threshold (ESRT) tile electrode arrangementfor sensing electrical activity in target tissue such as the stapediusmuscle. An inner electrode has an elongate electrode body formed as acylindrical section with an inner penetrating end for insertion into thetarget tissue. An outer electrode fits over the inner electrode and hasan outer penetrating end for insertion into the target tissue. The twoelectrodes are joined together with their electrode bodies in parallelso that the penetrating ends of the electrodes penetrate in the samedirection into the target tissue to sense electrical activity in thetarget tissue.

The electrodes may be arranged to have the outer penetrating endpenetrate into the target tissue before the inner penetrating end, or tohave the inner penetrating end penetrate into the target tissue beforethe outer penetrating end. There may also be an insulation layerarranged between the electrodes to electrically isolate the electrodesfrom each other. The arrangement may include an electrically elicitedstapedius reflex threshold (ESRT) sensing arrangement.

Embodiments of the present invention also include an electrode fixationstructure for securing an implantable electrode sensing arrangement. Animplantable fixation bar of deformable material has attachment openingsat opposing ends for securing the opposing ends to underlying bone. Alead holder is connected to the fixation bar for holding wire leads ofthe electrode sensing arrangement in a defined position.

The lead holder may be based on a ring shape. The fixation bar may bemade of a deformable metal material. The implantable electrode sensingarrangement may include an electrically elicited stapedius reflexthreshold (ESRT) sensing arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrode arrangement for sensing electrical activity intarget tissue as is known in the prior art.

FIG. 2 A-C shows an Evoked Stapedius Reflex Threshold (ESRT) TileElectrode according to one embodiment of the present invention.

FIG. 3 A-B shows an electrode lead holder according to one embodiment ofthe present invention.

FIG. 4 A-C shows assembly of an ESRT Tile electrode according to oneembodiment of the present invention.

DETAILED DESCRIPTION

Various embodiments of the present invention are directed to an EvokedStapedius Reflex Threshold (ESRT) Tile Electrode arrangement for sensingelectrical activity in the stapedius muscle. Two matching half shellcylindrical section electrode structures have parallel electrode bodieswith penetrating ends of the electrodes that penetrate into thestapedius muscle in the same direction. An electrode fixation structurecan secure the electrode arrangement in place.

For example, FIG. 2 A-C shows one specific embodiment of an ESRT TileElectrode 200. An inner electrode 202 has an elongate electrode bodyformed as a cylindrical section with an inner penetrating end 205 forinsertion into the target tissue. An outer electrode 201 fits over theinner electrode 202 and also has an outer penetrating end 204 forinsertion into the stapedius muscle target tissue. In order to optimizethe active electrical surface of penetrating ends 204 and 205, a roughsurface such as a fractal coating may be useful to provide for a properrecording over a prolonged implantation time which may include tissuegrowth over the electrode surfaces and consequent lowering of the phaseinterface impedance.

The rear part of each electrode 201 and 202 forms a flat thin tail whichprovides a stable elongation for surgical handling, allowing, forexample, fixation of the Tile Electrode 200 in the posterior tympanotomyby bending the elongated tail portion of the electrodes 201 and 202 intoan appropriate position with respect to the bony bridge. The tails ofthe electrodes 201 and 202 also provides a good location for connectingto the electrode wires 207 back to the implant processor (stimulator).

The two electrodes 201 and 202 are joined together with their electrodebodies in parallel so that the penetrating ends 204 and 205 penetrate inthe same direction into the target tissue. In the embodiment shown inFIG. 2, the outer penetrating end 204 penetrates into the target tissuebefore the inner penetrating end 205. In other embodiments, it may bethe other way round with the inner penetrating end 205 penetrating intothe target tissue before the outer penetrating end 205. In the specificembodiment shown in FIG. 2, the penetrating ends 204 and 205 are axiallydisplaced from each other by around one third of the total length of theelectrodes 201 and 202, allowing them to behave as two separateindependent electrodes.

Both electrodes 201 and 202 can be radially displaced from each other aswell as longitudinally to come up with a proper EMG recording. Forexample, in the embodiment shown in FIGS. 2B and 2C, there also is aninsulation layer 206 arranged between the electrodes 201 and 202 toelectrically isolate them from each other. The resulting radial spatialdisplacement of the two electrodes 201 and 202 allows for bipolarrecordings.

The arrangement of the Tile Electrode 200 allows an easy and quicksurgical placement at the stapedius tendon, which then can be moved intothe stapedius muscle. The compact and robust design of the TileElectrode 200 enables the simultaneous insertion of the penetrating ends204 and 205 of both electrodes 201 and 202 into the stapedius muscletarget tissue despite the small size of the muscle itself. Compared tothe prior art electrode arrangement shown in FIG. 1, the present TileElectrode 200 does not require any penetration holes in the electrodes201 and 202, nor does surgical implantation of the Tile Electrode 200require the complicated insertion of a fixation electrode perpendicularto the main electrode body, thereby saving considerable time during thesurgical implantation of the device.

FIG. 3 A-B shows an electrode lead holder 300 according to oneembodiment of the present invention which acts as an electrode fixationstructure to facilitate securing of the electrode sensing arrangement ina desired fixed position within the middle ear. In the embodiment shownin FIG. 3, an implantable fixation bar 302 is made of some deformablematerial such as soft metal so it can be bent as desired during surgicalimplantation. The fixation bar 302 includes attachment openings 303 atopposing ends for securing the fixation bar 302 to underlying bone. Alead holder 301 is connected to the fixation bar 302 for holding wireleads of the electrode sensing arrangement in a defined position. In theembodiment shown in FIG. 3, the lead holder 301 is in the specific formof a cylindrical ring.

During surgery to implant the electrode, the surgeon would place theelectrode lead holder 300 first, attaching it firmly to a middle earbony structure (such as the bone bridge in front of the incus, or on theincus, or on the middle ear wall such as on the promontory). Theelectrode lead holder 300 avoids placing tension on the connectingelectrode wires and on the stapedius tendon and/or muscle, thereby helpsto keep the electrode structure in place.

When the electrode lead holder 300 has been brought into properposition, the electrode sensing arrangement then can be adjusted to itsfinal position. Once the arrangement is installed, the electrode leadholder 300 can permanently hold the electrode sensing arrangement in itsimplanted position to reliably record signals over several months oryears. In some embodiments, the electrode lead holder 300 may also serveas a reference electrode or as a second sensing electrode where themuscle/tendon electrode is a single electrode.

The fixation structure of the electrode lead holder 300 helps avoiddamage to the electrode sensing arrangement during implantation: Theelectrode lead holder 300 can be manipulated with a forceps or anothersurgical instrument and thus allow the surgeon to bring it intoposition. This can occur without direct contact of the surgeon'sinstrument to the more delicate electrode structure which is thereforeless likely to be damaged by the surgeon's instrument.

FIG. 4 A-C shows assembly of an ESRT Tile electrode according to oneembodiment of the present invention. Initially the inner electrode 402on the left side of FIG. 4A and the outer electrode 401 on the rightside are separate structures. Side wings 403 are bent into a curveadapted to fit around the target tissue (muscle or tendon) where theelectrode is ultimately to be placed. Then the outer top side of theinner electrode 402 (the back side in this view) and/or the inner bottomside of the outer electrode 401 (the front side in this view) aresprayed with a layer of insulation material 406, e.g., silicone, and thetwo electrodes 401 and 402 are fit together over the wet insulation,FIG. 4B. The insulation material 406 is cured for a period, e.g., 30minutes at 120° C., then more insulation material 406 may be applied tothe remainder of the electrode and cured. Portions of the innerelectrode 402 and outer electrode 401 may be masked while the insulationmaterial 406 is applied. After assembly and insulation, the masking maybe removed to expose bare metal from desired parts of the electrodes 401and 402 such as the penetrating ends 404 and 405 and/or the tail pieceswhere connecting wires 407 may be soldered or welded on.

Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention.

1. An electrode arrangement for sensing electrical activity in targettissue, the arrangement comprising: an inner electrode having anelongate electrode body formed as a cylindrical section with an innerpenetrating end for insertion into the target tissue; and an outerelectrode fitting over the inner electrode and having an outerpenetrating end for insertion into the target tissue; wherein theelectrodes are joined together with their electrode bodies in parallelso that the penetrating ends penetrate in the same direction into thetarget tissue to sense electrical activity in the target tissue.
 2. Anelectrode arrangement according to claim 1, wherein the electrodes arearranged to have the outer penetrating end penetrate into the targettissue before the inner penetrating end.
 3. An electrode arrangementaccording to claim 1, wherein the electrodes are arranged to have theinner penetrating end penetrate into the target tissue before the outerpenetrating end.
 4. An electrode arrangement according to claim 1,further comprising: an insulation layer arranged between the electrodesto electrically isolate the electrodes from each other.
 5. An electrodearrangement according to claim 1, wherein the arrangement comprises anelectrically elicited stapedius reflex threshold (ESRT) sensingarrangement.
 6. An electrode fixation structure for securing animplantable electrode sensing arrangement, the structure comprising: animplantable fixation bar of deformable material; attachment openings atopposing ends of the fixation bar for securing the opposing ends tounderlying bone; and a lead holder connected to the fixation bar forholding wire leads of the electrode sensing arrangement in a definedposition.
 7. An electrode fixation structure according to claim 5,wherein the lead holder is based on a ring shape.
 8. An electrodefixation structure according to claim 5, wherein the fixation bar ismade of a deformable metal material.
 9. An electrode fixation structureaccording to claim 5, wherein the implantable electrode sensingarrangement includes an electrically elicited stapedius reflex threshold(ESRT) sensing arrangement.